Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device. More particularly, the invention relates to imaging lenses which are built in image pickup devices mounted in highly functional products such as smart TVs and 4K TVs, information terminals such as game consoles and PCs, and mobile terminals such as smart phones, mobile phones and PDAs (Personal Digital Assistants).
Description of the Related Art
In recent years, highly functional products, such as a smart TV as a TV with a personal computer function and a 4K TV as a TV with four times higher resolution than a full high-definition TV, have been attracting attention. As for smart TVs, there has been a tendency towards more multifunctional models such as ones which incorporate an image pickup device capable of taking a high resolution image and can transmit the image through a communication network, in addition to the high functionality. Also, due to its high resolution, a 4K TV can reproduce an image which is so realistic as if the object were there. These products are expected to provide a wider range of functions than before: for example, a security function combined with a high-accuracy face recognition function and a moving body detection function, a pet monitoring function, and a function of editing an image captured by image processing technology in various ways. If these products have an ability to take higher resolution images or moving images, they are expected to become products which increase the satisfaction of the general consumers. On the other hand, recently introduced into the market are smart phones which use an image sensor with a resolution of more than 40 megapixels to provide a professional quality digital camera function. Therefore, there is a growing demand for higher camera performance for these products.
However, in the conventional techniques, it is difficult to provide an imaging lens which satisfies the performance requirement of the devices as mentioned above. For example, the image pickup device used in a smart TV or smart phone with a high resolution imaging function is assumed to adopt a relatively large image sensor suitable for high resolution images. In that case, since a larger image sensor is used, there arises the following problem: the optical system should be larger, so it is difficult to correct various aberrations and it is impossible to maintain the high optical performance achieved so far with a conventional smaller image sensor. In addition, in the case of a monitoring camera, the following problem may arise: the camera is required to use a wide-field of view imaging lens and when the lens is designed to provide a wide field of view, correction of aberrations may be very difficult particularly in the peripheral area regardless of image sensor size and it may be impossible to deliver satisfactory optical performance.
Furthermore, when an imaging lens is used in an image pickup device with an autofocus function which is recently popular, high optical performance must be ensured in both imaging of an object at infinity and imaging of an object at close range, but this is very difficult particularly when the image sensor in use is large. In addition, for mobile terminals including smart phones, the imaging lenses must be always compact enough to meet the product design need.
As an imaging lens built in an apparatus with an image pickup device, the imaging lens described in JP-A-2010-262270 (Patent Document 1) or the imaging lens described in JP-A-2012-155223 (Patent Document 2) is known.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first lens with positive refractive power having a convex shape on the object-side surface near an optical axis, a second lens with negative refractive power, a third lens with positive refractive power having a concave shape on an image-side surface near the optical axis, a fourth lens with positive refractive power having a convex shape on the image-side surface near the optical axis, and a fifth lens with negative refractive power near the optical axis. The imaging lens described in Patent Document 1 includes five constituent lenses (elements), each of which is optimized to deliver high performance.
Patent Document 2 discloses an imaging lens which includes, in order from an object side, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with negative refractive power, a fifth lens group with positive refractive power, and a sixth lens group with negative refractive power. In the imaging lens described in Patent Document 2, the lens configuration of the optical system is concentric with an aperture stop so as to suppress astigmatism and coma aberrations and achieve a wider field of view.
In the imaging lens described in Patent Document 1, five constituent lenses are used for higher performance and the lens system provides a relatively wide field of view with a half field of view of about 38 degrees. However, its ability to correct aberrations with the five constituent lenses is limited and insufficient to respond the recent demand for higher resolution.
According to the use of six lens groups, the imaging lens described in Patent Document 2 provides a relatively wide field of view and can correct aberrations properly. However, in imaging of an object at infinity and imaging of an object at close range, a specific lens group must be moved in the optical axis direction for focusing, so there is a problem that the structure is complicated. Also, if the lens configuration described in Patent Document 2 is employed to provide a wide field of view, correction of aberrations will be difficult particularly in the peripheral area and high optical performance cannot be delivered.
As stated above, in the conventional techniques, it is difficult to provide a compact imaging lens which can take a high resolution image as demanded in recent years with a wide field of view and delivers high performance in both imaging of an object at infinity and imaging of an object at close range.